As integrated circuit (IC) device geometries shrink, development of low-k inter-metal dielectrics (IMDs) becomes more important. Current production processes use dense films that have a dielectric constant (k-value) between 3 and 4. These films are typically deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) processes and are typically made from either fluoro-silicate glass (FSG) or organo-silicate glass (OSG). Devices in current production have minimum feature sizes that range between 90 nm and 120 nm. The Semiconductor Industry roadmap calls for further shrinking these device geometries to 65 nm, then 45 nm and beyond over the remainder of this decade.
As device geometries shrink further, there is a need for IMD films with k-values under 2.7. Successfully developing a film of such low capacitance requires including porosity in the film. To this end, ultra-low-k (ULK) IMD films of porous OSG have been developed. These ULK films are deposited using PECVD techniques, wherein an OSG backbone and a pore generator (porogen) are co-deposited on a semiconductor wafer. Various techniques such as thermal, ultraviolet (UV) and electron beam curing are then used to drive the porogen out of the composite film leaving behind a porous OSG matrix. The resulting porous film exhibits k-values ranging from 2.0 to 2.5 due to the presence of pores containing air, which by definition has a k of 1.0.
However, the inclusion of pores in these films renders them softer and mechanically weaker than dense OSG films. Mechanical strength and hardness are necessary for the film to survive subsequent processing steps from chemical mechanical polishing to the various wire-bonding steps during chip packaging. Therefore, to compensate for the mechanical weakness introduced by the pores in these ULK films, the OSG backbone needs to be strengthened. Further processing of these wafers using UV radiation and electron beams increases cross-linking, which strengthens the film. Thermal curing has no further effect on the mechanical properties of the film after the porogen has been driven out, and therefore cannot be used to harden or strengthen the film.
Curing with commercially available mercury-vapor UV lamps often results in areas of non-uniformity on the wafer because the tubular geometry of these lamps is not optimized for uniform illumination of a wafer. Further, there are significant variations in UV output from lamp to lamp. Non-uniformity in pedestal and wafer temperature could cause non-uniformity in curing, and consequently in the properties of the porous ULK film.
Mercury-vapor lamps exhibit another significant shortcoming, namely that in order to generate said vapor, the lamps must operate at significantly higher temperatures than is desired for processing ULK films. To accelerate curing as much as possible, it is desirable that the wafer be processed as close to the maximum temperature limit (usually around 400° C.) as possible. Typical commercial lamp systems cause the lamp envelope to reach temperatures between 800° and 900° C. The IR radiation emitted by the lamp envelope is incident on the wafer in much the same way as the UV emanating from the lamp discharge. This will cause the wafer temperature to increase above the preferred set-point if active cooling is not performed. Moreover, wafer temperature non-uniformity due to lamp heating (typically 30° C. range) is far worse than that due to pedestal heating (as low as 3° C. range).
Therefore, there is a, need for inventions that improve the uniformity of curing and wafer throughput by heating the semiconductor wafer to its operating temperature (e.g., 400° C.), while removing the IR heat flux incident on the wafers from the lamps. It would also be desirable to remove heat from the pedestal using commonly available facility cooling water.